1. Field of the Invention
The present invention relates to an optical disk recording/playback apparatus and optical disk evaluation method that illuminates an optical disk with laser light, receives the laser light modified by pits or marks recorded on the optical disk, and converts the light amount of the received laser light into an electrical signal, thereby obtains a playback signal to evaluate the optical disk.
2. Description of the Related Art
To date, an evaluation apparatus called a jitter meter has been used as an evaluation apparatus for optical disks. See for example Japanese Patent Laid-open Publication No. 11-167720. This evaluation apparatus quantitatively measures temporal variation called jitter in the playback signal of an optical disk. The dedicated jitter meter is expensive, and jitter cannot have been evaluated readily and inexpensively. Accordingly, a method of evaluating jitter by using an optical disk recording/playback apparatus has been come up with.
FIG. 20 shows a CD recording/playback apparatus 100 as an optical disk recording/playback apparatus having a function of evaluating optical disks. The operation of playing back an optical disk 2 by this CD recording/playback apparatus 100 will be described.
A pickup 1 receives reflected light from optical disk 2 illuminated with light and converts a high/low intensity of the reflected light into a high/low level of voltage. A pickup controller 3 controls the reading position of pickup 1 relative to optical disk 2 such that pickup 1 can read out, in correct order, data represented by pits or marks recorded on optical disk 2. A binarizing circuit 4 reads changes in voltage output from the pickup 1 and produces an EFM (Eight to Fourteen Modulation) signal having one frame of 588 bits as a unit. This EFM signal has a repetitive pattern of High and Low levels. The time periods of these High and Low levels vary between 3T and 1T, nine different time periods, where T is the time period of 1 bit that is about 230 ns.
A digital signal processing circuit 5 performs EFM demodulation on the EFM signal output from the binarizing circuit 4, and performs CIRC (Cross-Inter leave Reed-Solomon Code) decoding on the demodulated EFM signal to produce CD-ROM data having frames of 24 bytes. A CD-ROM decoder 6 detects for read errors in the CD-ROM data inputted from the digital signal processing circuit 5, corrects the errors, and outputs the corrected CD-ROM data to a host computer.
A buffer RAM 7 is connected to the CD-ROM decoder 6, and temporarily stores CD-ROM data inputted from the digital signal processing circuit 5 to the CD-ROM decoder 6, on a block unit basis. Because error correction is performed on one block of data, the process in the CD-ROM decoder 6 needs at least one block of CD-ROM data. CD-ROM data is sequentially read out, and one block of CD-ROM data necessary for each process execution is stored in the buffer RAM 7. The buffer RAM 7 is a DRAM in order to store a large amount of data. A control microcomputer 8 is constituted by a so-called one-chip microcomputer having a ROM and a RAM incorporated, controls the operation of the CD-ROM decoder 6 according to a control program recorded in the ROM, and at the same time, temporarily stores command data input from the host computer and sub-code data input from the digital signal processing circuit 5 in the incorporated RAM. Thus, the control microcomputer 8 controls the operation of each component in response to instructions from the host computer, to make the CD-ROM decoder 6 output desired CD-ROM data to the host computer.
Next, a method of evaluating jitter of optical disk 2 in CD recording/playback apparatus 100 will be described.
The pickup 1, optical disk 2, pickup controller 3, and binarizing circuit 4 are controlled by the control microcomputer 8 to operate in the same way as in the playback operation for optical disk 2, but the digital signal processing circuit 5 and CD-ROM decoder 6 are controlled by the control microcomputer 8 not to operate, and the buffer RAM 7 operates in a different way as in the playback operation.
A counter 10 is connected to the binarizing circuit 4, and reads in the EFM signal output from the binarizing circuit 4. The counter 10 counts counter clocks of higher frequency than that of the input EFM signal from each change point of the polarity of the EFM signal to the next change point (i.e., counts each time period indicating “High” or “Low” level), and writes the count values sequentially into the buffer RAM 7. For a CLV operation of constant linear velocity at single speed, the 1T of the EFM signal is about 230 ns, and hence, if counter clocks having a clock period of 2 ns, higher in frequency, are used in counting, the count value for clock period 3T of the EFM signal that is about 690 ns (about 230 ns×3) is 345 ideally. Likewise, the count value for clock period 4T of the EFM signal is 460, the count value for clock period 5T is 575, . . . , the count value for clock period 11T is 1265. After performing this operation on a given area of data recorded on the optical disk 2, the control microcomputer 8 evaluates jitter by analyzing the count values stored in the buffer RAM 7.
Here, the count values, measured data, are written on a word (16 bits) unit basis from the counter 10 into the buffer RAM 7 constituted by a DRAM as shown in FIG. 21. The counter 10 writes one count value as measured data by outputting five commands, ACT (active), NOP (no operation), WRIT (write; input=DATA1), PRE (pre-charge), and NOP (no operation), with respect to the basic clock to the buffer RAM 7. That is, it takes 5 cycles of the basic clock to write one count value as measured data.
The writing of measured data in the CD recording/playback apparatus 100 will be described in detail using FIG. 22. The counter 10 continuously reads in the EFM signal from the binarizing circuit 4, resets its count value each time the polarity of the EFM signal changes from High to Low or from Low to High, and counts counter clocks higher in frequency than the EFM signal during each EFM clock period of the EFM signal. Then, the next time when the polarity of the EFM signal changes from Low to High or from High to Low, the counter 10 stores the count value up to here in a register of the counter 10 and resets its count value. Then, while counting during the next EFM clock period, the counter 10 writes the count value for the preceding EFM clock period stored in the register into the buffer RAM 7. A memory management circuit incorporated in the counter 10 dedicatedly performs the writing into the buffer RAM 7 and outputs commands to write into buffer RAM 7. In this way, the counter 10 counts during the current EFM clock period, while writing the count value for the preceding EFM clock period into the buffer RAM 7. It takes 5 cycles of the basic clock, time period T1, for the counter 10 to write the count value for the preceding EFM clock period as measured data into the buffer RAM 7.
The counter 10 first resets its count value when the polarity of the EFM signal changes. The next time when the polarity of the EFM signal changes, the counter 10 stores the count value N1 in the register and resets its count value.
Then, while counting counter clocks during the second EFM clock period, the counter 10 writes the count value N1 stored in the register into the buffer RAM 7 in the time period T1. The next time when the polarity of the EFM signal changes, the counter 10 stores the count value N2 in the register and resets its count value. Then, while counting counter clocks during the third EFM clock period, the counter 10 writes the count value N2 stored in the register into the buffer RAM 7 in the time period T1. By repeating this operation, the counter 10 writes sequentially the count values for EFM clock periods into the buffer RAM 7.
However, in the conventional art, it takes the predetermined time period T1 to write the count value for the preceding EFM clock period into the buffer RAM 7, and if the polarity of the EFM signal changes during that time period, lack of the count value may occur in the measured data that the counter 10 writes into the buffer RAM 7. In an example shown in FIG. 22, while the counter 10 is writing the count value N3 for the third EFM clock period into the buffer RAM 7, the polarity of the EFM signal changes. Thus, measured data of the count value N4 is not written into the buffer RAM 7, and thus, the problem occurs that jitter cannot be accurately evaluated.